Sample Required?

Test Preparation Needed?

You will be instructed to empty your bladder and bowels prior to sample collection. It will be necessary to lie still in a curled-up fetal position during the test and to lie quietly for a time period after the collection.

The Test Sample

What is being tested?

Cerebrospinal fluid (CSF) is a clear watery liquid that is formed and secreted by the choroid plexus, a special tissue that has many blood vessels and that lines the small cavities or chambers (ventricles) in the brain. CSF flows around the brain and spinal cord, surrounding and protecting them. It is continually produced, circulated, and then absorbed into the blood system. About 17 ounces (500 mL) are produced each day. This rate of production means that all of the CSF is replaced every few hours. A CSF analysis is a group of tests that evaluate substances present in CSF in order to diagnose conditions affecting the central nervous system.

A protective blood-brain barrier separates the brain from the bloodstream and regulates the distribution of substances between the blood and the CSF. It helps keep large molecules, toxins, and most blood cells away from the brain. Any condition that disrupts this protective barrier may result in a change in the normal level or type of constituents of CSF. Because CSF surrounds the brain and spinal cord, testing a sample of CSF can be very valuable in diagnosing a variety of conditions affecting the central nervous system. Though a sample of CSF may be more difficult to obtain than, for example, urine or blood, the results may reveal more directly the cause of central nervous system symptoms.

Infections and inflammation in the meninges, the layers of tissue that surround the spinal cord and brain, can disrupt the blood-brain barrier and allow white blood cells (WBCs) and red blood cells (RBCs) and increased amounts of protein into the CSF. Meningitis, an infection in the meninges, and encephalitis, an infection in the brain, can also lead to the production of antibodies. Immune diseases that affect the central nervous system, such as Guillain-Barré Syndrome and multiple sclerosis, can also produce antibodies that can be found in the CSF. Cancers such as leukemia can lead to an increase in CSF white blood cells, and cancerous tumors can result in the presence of abnormal cells. These changes from normal CSF constituents make the examination of cerebrospinal fluid valuable as a diagnostic tool.

CSF analysis usually involves an initial basic set of tests performed when CSF analysis is requested:

A wide variety of other tests may be ordered as follow-up depending on the results of the first set of tests. The specific tests that are ordered may also depend on the signs and symptoms a person has and the disease the doctor suspects may be the cause. Each of these tests can be grouped according to the type of exam that is performed:

Chemical tests —this group refers to those tests that detect or measure the chemical substances found in spinal fluid. Many of the substances in CSF are also in blood and the relative amounts in CSF and blood are often compared. Normally, levels of certain constituents of CSF, such as protein and glucose, are reflective of their concentration in the blood.

Microscopic examination (Cell count and differential)—any cells that may be present are counted and identified by cell type under a microscope.

How is the sample collected for testing?

A sample of cerebrospinal fluid (CSF) is collected by a doctor from the lower back using a procedure called a lumbar puncture or spinal tap. Often, three or more separate tubes of CSF are collected, and multiple tests may be run on the different samples.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

The person being tested should empty their bladder and bowels prior to the sample collection. It will be necessary to lie still in a curled-up fetal position during the test and to lie quietly for a time period after the collection.

When is it ordered?

CSF analysis may be ordered when a doctor suspects that a person has a condition or disease involving their central nervous system. A person's medical history may prompt the request for CSF analysis. It may be ordered when someone has suffered trauma to the brain or spinal cord, has been diagnosed with cancer that may have spread into the central nervous system or has signs or symptoms suggestive of central nervous system involvement.

The signs and symptoms of central nervous system conditions can vary widely and many overlap with a variety of diseases and disorders. They may have sudden onset, suggesting an acute condition, such as central nervous system bleeding or infection, or may be slow to develop, indicating a chronic disease, such as cancer or multiple sclerosis.

Depending on a person's history, a doctor may order CSF analysis when some combination of the following signs and symptoms appear:

Changes in mental status and consciousness

Sudden, severe or persistent headache or a stiff neck

Confusion, hallucinations or seizures

Muscle weakness or lethargy, fatigue

Nausea (severe or prolonged)

Flu-like symptoms that intensify over a few hours to a few days

Fever or rash

Sensitivity to light

Numbness or tremor

Dizziness

Difficulties with speech

Difficulty walking, lack of coordination

Mood swings, depression

Infants may be irritable, cry when they are held, have body stiffness, refuse food, and have bulging fontanels (the soft spots on the top of the head)

What does the test result mean?

CSF usually contains a small amount of protein and glucose and may have a few white blood cells.

Any condition that disrupts the normal pressure or flow of CSF or the protective ability of the blood/brain barrier can result in abnormal results of CSF testing. For detailed explanations of what various test results may mean, see the sections on:

Is there anything else I should know?

Bacterial and parasitic (such as amebic) meningitis are medical emergencies. The doctor must rapidly distinguish between these conditions, viral meningitis, which is typically milder, and other conditions with similar symptoms. Because prompt treatment is crucial, the doctor may start the person affected on a broad-spectrum antibiotic before the diagnosis has been definitely determined.

To help diagnose a central nervous system-related illness, the doctor may want to know about recent vaccinations, sickness, contact with others who are ill, places a person has traveled to, what symptoms a person is experiencing, and their duration.

1. What is a lumbar puncture (spinal tap) and how is it performed?

The lumbar puncture is a relatively routine procedure. It is usually performed while you are lying on your side in a curled up fetal position but may sometimes be performed in a sitting position. It is important that you remain still during the procedure. Once you are in the correct position, your back is cleaned with an antiseptic and a local anesthetic is injected under the skin. When the area has become numb, a special needle is inserted through the skin, between two vertebrae, and into your spinal canal. An "opening" or initial pressure reading of the CSF is obtained. The doctor then collects a small amount of CSF in multiple sterile vials. A "closing" pressure is obtained, the needle is withdrawn, and a sterile dressing and pressure are applied to the puncture site. You will then be asked to lie quietly in a flat position, without lifting your head, for one or more hours to avoid a potential post-test headache.

The lumbar puncture procedure usually takes less than half an hour. For most patients, it is a moderately uncomfortable procedure. The most common sensation is a feeling of pressure when the needle is introduced. Let your doctor know if you experience a headache or any abnormal sensations, such as pain, numbness, or tingling in your legs, or pain at the puncture site.

The lumbar puncture is performed low in the back, well below the end of the spinal cord. There are spinal nerves in the location sampled, but they have room to move away from the needle. There is the potential for the needle to contact a small vein on the way in. This can cause a "traumatic tap," which just means that a small amount of blood may leak into one or more of the samples collected. While this is not ideal, it may happen a certain percentage of the time. The evaluation of your results will take this into account.

2. Are there other reasons to do a lumbar puncture?

3. Why do I need a spinal tap? Why can't my blood or urine be tested?

Spinal fluid, obtained during a spinal tap, is often the best sample to use for conditions affecting your central nervous system because your CSF surrounds your brain and spinal cord. Changes in the elements of your CSF due to central nervous system diseases or other serious conditions are often first and most easily detected in a sample of your spinal fluid. Tests on blood and urine may be used in conjunction with CSF analysis to evaluate your condition.

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Article Sources

NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.

Sur, D. and Bukont, E. (2007 June 15). Evaluating Fever of Unidentifiable Source in Young Children. American Family Physician v 75 (12). Available online through http://www.aafp.org/. Accessed on 3/18/08.

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This article was last reviewed on January 12, 2012. | This article was last modified on December 29, 2014.

The review date indicates when the article was last reviewed from beginning to end to ensure that it reflects the most current science. A review may not require any modifications to the article, so the two dates may not always agree.

The modified date indicates that one or more changes were made to the article. Such changes may or may not result from a full review of the article, so the two dates may not always agree.